The present invention relates to apparatus and a method for reducing the crest factor of a signal, such as, for example, an Orthogonal Frequency Domain Multiplexing (OFDM) signal.
OFDM is widely used in wireless systems such as Third Generation Partnership Project Long Term Evolution (3GPP LTE), Wimax and Digital Video Broadcasting (DVB). OFDM signals used in these systems typically have a high Peak to Average Power Ratio (PAPR). The high PAPR of the OFDM signals increases the performance requirements on power amplifiers used in such receivers; either a highly linear power amplifier is required or some of the power efficiency of the power amplifier must be sacrificed to meet the PAPR requirement. High performance power amplifiers are costly, increasing the capital expenditure of the receiver, whilst reducing the efficiency of the power amplifier is undesirable, as low efficiency power amplifiers require additional components for cooling, whilst also increasing the operational expenditure of the receiver and wasting energy, since they consume more power than high efficiency amplifiers.
In an attempt to alleviate these problems, a crest factor reduction (CFR) block is sometimes introduced in OFDM transmitters upstream of the power amplifier to reduce the PAPR of an input signal, thereby reducing the linearity requirement of the power amplifier, allowing a lower specification (and therefore lower cost) amplifier to be used in the receiver. The CFR stage typically operates by clipping the input signal such that its amplitude envelope falls below a threshold within which the power amplifier can operate satisfactorily. Increasing pressure to reduce the cost of OFDM systems has led manufacturers to consider using lower cost components, such as lower performance amplifiers. Existing CFR blocks cannot be used in systems which employ such lower performance amplifiers, as they cannot produce an output signal with a PAPR which is low enough for satisfactory performance of the power amplifier whilst also guaranteeing an acceptable error vector magnitude (EVM) in the system.